CODEWITHSUNDEEP
objective-cmacoscocoaimage-capture
Sankalp
Sankalp

Reputation: 2824

How to instantiate a pointer in objective-c

Suppose I have a pointer to type ICScannerDevice class for which I have allocated some memory. Now, I want to initialize it with the pointer of type ICDevice which is a parent class of ICScannerDevice. How do I do this?

Is the following the correct method? I guess not!

ICScannerDevice* scannerDevice = [[ICScannerDevice alloc] init];
scannerDevice = (ICScannerDevice*)device; // device is a pointer to ICDevice.

Upvotes: 0

Views: 828

Answers (3)

CRD
CRD

Reputation: 53010

Suppose I have a pointer to type ICScannerDevice class for which I have allocated some memory.

ICScannerDevice* scannerDevice = [[ICScannerDevice alloc] init];

You are mixing two things up here. Variables are created by declarations, a declaration can also contain an initialisation expression.

When you declare a variable space is allocated which can contain a value of the variable's type - whether that type is int, in which case the value would be a integer e.g. 42, or ICScannerDevice *, in which case that value would be a reference to an allocated ICScannerDevice object.

So the declaration:

ICScannerDevice* scannerDevice

provides all the space you need to store a value of type ICScannerDevice *. The initialisation expression you give:

[[ICScannerDevice alloc] init];

is in your case redundant (and wasteful), you do not wish to create an object of type ICScannerDevice and store a reference to that object in scannerDevice as you wish to store a totally different reference in your variable:

Now, I want to initialize it with the pointer of type ICDevice which is a parent class of ICScannerDevice.

scannerDevice = (ICScannerDevice*)device; // device is a pointer to ICDevice.

This is wrong as you state that device holds a reference to an ICDevice object. As the type ICDevice is a parent of ICScannerDevice a reference to an ICScannerDevice can be treated as one to an ICDevice - aka "upcast".

However a reference known to be an object of type ICDevice may or may not be to an ICScannerDevice it might be , say, to an ICPlotterDevice. So you cannot directly cast - aka "downcast" and assign.

You must first check you actually have a reference to an ICScannerDevice. You do this by checking whether the type of your referenced object, using method isKindOfObject:, is the type ICScannerDevice, and you obtain that type with the class method:

if ( [device isKindOfObject:[ICScannerDevice class]] )
{
   // we have a reference to an ICScanner Device
   scannerDevice = (ICScannerDevice *)device;  // downcast and assign
    ...
}
else
{
   // device references something other than an ICScannerDevice object
   // handle this case
   ...
}

You also need to handle the case where the value stored in device is nil - i.e. it references nothing. You might just want to combine this with the test in the if and treat it as an ICScannerDevice reference:

if ( (device == nil) || [device isKindOfObject:[ICScannerDevice class]] )

or you may need to handle it some other way.

HTH

Upvotes: 3

Phillip Mills
Phillip Mills

Reputation: 31016

First, there's no point in allocating an object if you're immediately going to assign some other object to its reference.

Second, if device is not a ICScannerDevice, casting it as one is an error that may or may not cause you problems depending on what you do with it next.

So, the answer is: no, it's not really correct.

(To give you a better answer about what you should be doing instead would require a little more context about where device comes from and how you want to use scannerDevice.)

Upvotes: 1

Daij-Djan
Daij-Djan

Reputation: 50109

a pointer is a number that holds a memory address where the object is.

a pointer is(16/32/64 bit) and lives on the stack. just its content is on the heap.

therefore, you don't need to allocate any space for a pointer. Only for the content. Since - in your case - you get a pointer to existing content. Just copy the memory address:

ICScannerDevice* = (ICScannerDevice*)device; // device is a pointer to ICDevice.

NOTE: remember casts don't change the content! if device isn't really a ICScannerDevice but something else, that might crash later.

Upvotes: 1

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